Specifying the number of ISOs that can be issued

Hey all! This question is for my attorney-friends here, or anyone else familiar with Section 422 -

I just recently found out that the number of ISOs that can be issued from a plan must be specified in the equity incentive plan documents, in order for ISOs to be issued from an equity plan compliantly. Does the specific number of ISO shares need to be specified in the plan document, or can it be written that ISOs can be issued up to 80% (for example) of the total shares in the plan?

And if the specific number of ISO shares that can be issued must be specified, does that number need to be reviewed and potentially increased with each plan amendment (assuming the new ISO grants will push the number over the initial share count that was specified)?

Anything else important to know about this, in regards to issuing ISOs compliantly?